Add analyzer rule template

.gitlab/issue_templates/Add style proposal

+## Description of the proposal
+
+<!--
+Please describe the proposal and add a link to the source (for example, http://www.betterspecs.org/).
+-->
+
+- [ ] Mention the proposal in the next backend weekly call and the #backend channel to encourage contribution
+- [ ] Proceed with the proposal once 50% of the maintainers have weighed in, and 80% of the votes are :+1:
+- [ ] Once approved, mention it again in the next backend weekly call and the #backend channel
+
+
+/label ~"development guidelines"
+/label ~"Style decision"
+/label ~Documentation
+
+/cc @gitlab-org/maintainers/rails-backend